不同阳离子影响下小麦根吸收镉的动力学过程

生物配体模型(BLM)是基于稳定态假设的一个平衡模型,能够较准确地运用于预测重金属的毒性及其生物有效性.根据BLM模型的假设条件模拟土壤溶液,研究了Ca2+、Mg2+、Cu2+和Ni2+对小麦根吸收Cd2+动力学的影响,阐述了BLM在应用中存在的局限性.结果表明:在8h内的短期动力学吸收实验中,小麦根对Cd的吸收及吸附量呈线性增加,但未达到稳定值,这说明Cd2+在根-液界面未达到稳定平衡;并且小麦根对Cd的吸收是受溶液中Cd2+扩散速率影响的.小麦根对Cd的吸收通量与Cd2+浓度Cd2+]和Cd与小麦根表上的配体结合浓度{Cd-Rcel}l都具有较好的线性关系,但随着Cd2+浓度的增加稳定常数KM-Rcell、渗透常数p和同化速率常数kint均呈降低趋势.Ca2+、Mg2+和Cu2+降低了小麦根对Cd2+的吸收通量,而Ni2+对Cd2+的竞争作用不显著.由于Cd与Ca、Ni有共同的吸收通道,Ca通道吸收达到饱和后可降低小麦根对Cd的吸收,但是Ca2+、Mg2+和Cu2+通过降低内化速率常数也可使小麦根对Cd的内化通量减少,因此不能完全用竞争效应解释其结果.另外,阳离子的存在会改变小麦根表的性质,Ca2+、Mg2+的存在会增加亨利系数KCd,即生物配体浓度{Rcel}l和(或)离子与生物配体结合稳定常数KCd-Rcell会增加,同时这些离子的存在还会降低渗透常数p和kint;Cu2+、Ni2+的存在会降低KCd;且p和k在低浓度Cd(0.01～0.05μM)条件下增加,在高浓度Cd条件下降低.

Kinetic Uptake of Cadmium by Wheat (Triticum aestivum) Roots Affected by Cations

Bioavailability of trace metals to plants is often accurately predicted by one of the steady-state models Terrestrial Biotic Ligand Model(t-BLM). According to the conditions of BLM model, the soil solution were prepared. A series of experiments concerning short-term kinetic biouptake of Cd2+ by wheat(Triticum aestivum)were carried out in the presence or absence of major cations(Ca2+ or Mg2+)or trace metals(Cu2+ or Ni2+)in soil solution to study the limitation of the application of BLM model. The results indicat that the biouptake of Cd in the root of wheat and the adsorbed Cd2+ on the surface of the root of wheat increased linearly with time for at least 8h, suggesting that the biouptake and adsorption of Cd2+ did not arrive the steady state in 8h. Moreover, the rate of diffusion of Cd2+ influenced on the uptake of the root of wheat. A linear relationship was established between the internalization flux of root of the wheat and the concentration of Cd2+ in solution(Cd2+])and the transporter-bound metal concentration{Cd-Rcell}. However, the stability constant KM-Rcell and the internalization rate constant kint decreased with increasing concentrations of Cd2+, indicating that the biouptake way was not single but lied at least two pathways(the high-affinity and the low-affinity way). Ca2+, Mg2+and Cu2+ in solution decreased the internalization flux of Cd2+, but the Ni2+ did not inhibit the uptake of Cd2+ in the root of wheat. This may be that the Ca2+ arriving at saturation in channels decrease the uptake of Cd2+ due to the Cd2+, Ca2+ and Ni2+ entering the cell via same channels. In addition, Ca2+, Mg2+ and Cu2+ could decrease the internalization flux of Cd2+ by the internalization rate constant kint. Therefore, the competing effect was not the only factor to explain the decreased internalization flux of Cd2+. Additionally, cations in solution changed the quality of the surface of the wheat root. Ca2+ and Mg2+ increased KCd and decreased the membrane permeability p and kint; Cu2+ and Ni2+ make the KCd decreased; p and kint of the wheat root decreased when the Cd concentration was at 0.5 or 1.0μM. The results suggests that the BLM should be improved to study the toxity and bioavailability of metals.